[time-nuts] Re: World's most accurate PC clock!
cfmd at bredband.net
Sun Jul 3 19:52:50 EDT 2005
From: Brooke Clarke <brooke at pacific.net>
Subject: Re: [time-nuts] Re: World's most accurate PC clock!
Date: Sun, 03 Jul 2005 16:09:15 -0700
Message-ID: <42C8701B.3070409 at pacific.net>
> Hi Tom:
> I've factored a number of the common crystal frequencies and have added
> their common applications, see:
A few notes:
27 MHz is "magic" since it is a common multiple to the NTSC and PAL line
systems. It is used in ITU-R BT.601 compatible video systems, the base
frequency for MPEG and MPEG-2 timing (that used a 90 kHz clock but went to full
27 MHz when they found they needed it, a bit kludgy but it works).
The DCF should be 77,5 kHz sharp. I found one ETSI document listing it at
77,5 MHz but that was a little to high. They got an editorial comment from me
on that one.
The 32,768 kHz is used in computers all over the place for their real-time
hardware clock. We had it since the IBM AT, so that was a definit fix for the
kludgy counter trick. Ah well.
As for your music references, there have existed a number of frequencies that
have been used, not all crystal oscillators. None of them is really correct.
The 4,33619 MHz "PAL" frequency is incorrect. It should be 4,43361875 MHZ
(+/- 5 Hz). Actually, that is for PAL B, D, G, H, I and N. Typo I'd assume.
1,544 MHz, 2,048 MHz, 19,44 MHz, 51,84 MHz and 155,52 MHz is standard telecom
frequencies. The two first ones are the traditional PDH rates (and thus
synchronisation frequencies), the third is the normal SDH/SONET reference
oscillator frequency, but 51,84 MHz and 155,52 MHz is more commonly used for
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